[codex] preserve explicit environment cwd

[pakrym-oai]

Why

TurnEnvironmentSelections::new rewrote the primary environment's explicit cwd to the legacy fallback cwd. For a remote-first selection, this could replace the remote working directory with a local fallback path and made the legacy cwd overlay authoritative over environment-owned state.

What changed

• Preserve every explicit environment cwd when constructing turn environment selections.
• Keep cwd-only app-server updates compatible by rebuilding the default environment selections at the requested cwd.
• Cover both explicit primary cwd preservation and cwd-only updates reaching the model-visible execution environment.

Testing

• just test -p codex-core session_update_settings_does_not_rewrite_sticky_environment_cwds
• just test -p codex-core environment_settings_preserve_explicit_primary_cwd
• just test -p codex-app-server thread_settings_update_cwd_retargets_default_environment

[chatgpt-codex-connector]

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[anp-oai]

LGTM, looks like this ate some bitrot and CI is unhappy but presumably an easy fix